1. Field of the Invention
The present invention generally relates to a multiple disc type impact absorbing device which absorbs an impact generated when a starter starts a driving operation of an engine.
2. Description of Related Art
When a starter starts a driving operation of an internal combustion engine, a driving torque of a motor is transmitted from an armature shaft to a ring gear of the engine through a pinion gear to crank the engine. At the cranking time of the engine, an impact force caused by a collision of the pinion gear with the ring gear is generated in the pinion gear and is transmitted to the armature shaft through a speed reducer. The starter has an impact absorbing device to absorb this impact force. For example, Published Japanese Patent First Publication No. 2005-113816 discloses a multiple disc type impact absorbing device of a starter.
In this device, a plurality of fixed discs and a plurality of rotary discs are alternately disposed so as to place each rotary disc between two fixed discs. The centers of the discs are placed on a common center axis. Each disc has an uneven surface acting as a frictional surface on each of both sides. A cone disc spring gives a pushing force to a laminator of the discs such that a static friction is generated between the frictional surfaces of each pair of fixed and rotary discs facing each other through an oil film of lubricant. Each rotary disc is fixedly joined to an internal gear of a planetary gear type speed reducer. Each fixed disc is supported by a case so as not to be rotated. When a load torque exceeding a slipping torque corresponding to the static friction is given from a pinion gear to the rotary discs through the internal gear as an impact force, each rotary disc slips on the adjacent fixed discs against the static friction and is rotated with the internal gear. Because the impact force is lost during the rotation of the rotary discs by sliding friction between the discs, the impact force is absorbed in the impact absorbing device to prevent the impact force from exerting an adverse influence on the reducer and/or the motor.
Each of the frictional surfaces of the fixed and rotary discs has a large number of dimples or concave portions formed by a stamping work, and lubricant is coated on the frictional surfaces and is held in the dimples. Therefore, the lubricant supplied from the dimples to a space between the frictional surfaces suppresses the generation of seizure on the frictional surfaces so as to prevent the discs from seizing up. Accordingly, the lubricant acts to prolong a life of the impact absorbing device.
When the rotary and fixed discs are punched by a pressing machine to form dimples on the frictional surfaces, the disc substance punched is moved as a fluid matter so as to rise up around each dimple. Therefore, a convex portion or a rising portion is formed around each dimple. In the device disclosed in the Publication, the dimples in each disc are arranged in a matrix shape at a fixed pitch along a radial direction of the disc and a circumferential direction perpendicular to the radial direction. The dimples in the rotary discs are placed in the radial direction at the same positions as the dimples of the fixed discs are placed.
With this arrangement of the dimples, when the rotary discs are rotated on the fixed discs, the dimples in each rotary disc are moved along the circumferential direction of the disc. Because the positions of the dimples in the rotary discs in the radial direction are the same as the positions of the dimples in the fixed discs, each dimple of the rotary discs passes on each of the corresponding dimples of the fixed discs placed in the radial direction at the same position as the position of the each dimple. Therefore, there is a high probability that the convex portion of each dimple of the rotary discs may periodically and directly come in contact with each of the convex portions of the corresponding dimples of the fixed discs without placing an oil film of lubricant between the convex portions of the dimples of the rotary and fixed discs. This direct contact increases abrasion between the convex portions. When this abrasion is continued, the convex portions are cut and planed, and flattened frictional surfaces of the rotary discs directly come in contact with those of the fixed discs. When this direct contact of the flattened frictional surfaces is frequently caused, a period of the direct contacts between the rotary discs and the fixed discs without any lubricant is lengthened.
As a result, the frictional surfaces of the discs partially seize up or are burned, and the slipping torque between the discs become unstable so as to change a rotational speed of the rotary discs with time. Finally, there is a high probability that the rotary discs may be locked due to the seizure of the discs. In other words, even when a driving torque is excessively transmitted to the rotary discs, the rotary discs cannot be rotated, and the impact absorbing device malfunctions.